P-T-XCO2 Pseudosection Modelling of Talc-Magnesite Soapstone

Abstract

Soapstone is considered to be a low metamorphic, carbonated ultramafic rock containing talc, magnesite, dolomite, chlorite and iron oxides; however in a broader sense of use including also non-carbonated compositions. Talc-magnesite soapstones are the mostly utilized in a furnace use, because they have well-known heat resistance properties. In the field talc-magnesite appears to be generated over serpentine, when a magnesium-rich ultramafic rock has been reacted with carbon dioxide-containing metamorphic fluids at greenschist to lower amphibolite facies conditions. In principle, the soapstone occurs in a wide pressure temperature range P = 0.5–10 kbar, T = 200–600 °C even with fluid of low CO2 ratios, though unaltered serpentinites are found to be as common in the same prospect areas. The object of this study is to define regional metamorphic carbonate-fluid phase equilibria of diverse ultramafic whole-rock compositions locating in East Finland and how to use it in the practical exploration work of magnesite soapstone formations. Talc-carbonate assemblage showing isochemical P-T and binary T-X equilibrium phase diagrams, i.e. pseudosectios are compiled by Theriak-Domino thermodynamic modelling program. Software is designed to calculate and plot the most correct stable assemblages for a given multi-component bulk chemical composition and case by case user-experimented H2O-CO2 fluid mixtures at prevailing pressure temperature condition. Unassisted calculating, general thermodynamic data sets utilizing program is based on Gibbs free energy minimization.